A very important aspect in the production of aluminum metal has not been mastered, namely preventing the formation of dross in molten aluminum. Due to its great affinity for oxygen, particularly at the higher temperature ranges reached in a furnace for melting or refining of aluminum, and on the surface where there is exposure to atmospheric oxygen, the metal returns to its oxide state forming dross. Dross is formed in the presence of even minor amounts of oxygen and this reaction is accelerated in the molten aluminum body in a furnace because of the high temperature. The aluminum oxide has higher melting temperature and lower specific gravity than molten aluminum; so aluminum furnace baths are normally covered with a layer of oxide solids. Skimming of this material from the bath invariably results in the coincidental removal of some of the underlying metal thus decreasing the potential yield.
Many efforts have failed to develop a method of converting this "secondary" aluminum oxide back to the metallic state once formed. Also, numerous attempts to develop a practice and economical method of recovering aluminum from this oxide material have been unsuccessful. Consequently, production of aluminum metal is accompanied by the formation of large quantities of aluminum dross as a waste product.
A recent attempt to resolve the dross problem in an aluminum furnace is set forth in U.S. Pat. No. 5,211,744, May 18, 1993 to L. A. Areaux, et al. for Method and Means for Improving Molten Metal Furnace Charging Efficiency. Therein, the dross formation is reduced by reducing oxygen in a trapped atmospheric blanket on the surface of the melt within a closed top furnace having a vacuum decontamination system removing gases from the aluminum surface. Inert gases are either produced by contaminants in the melt or introduced by an inert gas injector at or near the surface at the coolest point in the melt.
However this solution does not prevent formation of dross within the hottest molten aluminum region of the furnace where the formation of dross is accelerated, and furthermore requires an expensive and limited purpose special furnace with a vacuum operated decontamination system.
It is therefore an object of this invention to provide a feasible and low cost method to reduce dross formation in an aluminum furnace with the consequent great economical benefits in handling, dross processing and environmental problems.
Another object of the invention is to provide a method of reducing dross formation in an open top aluminum furnace in the absence of an atmosphere controlled decontamination system at the surface of the molten aluminum.
A still further object of the invention is to provide a more efficient and complete method of dross elimination in molten aluminum than heretofore available in the art.